This invention is directed to a very high contrast radiographic film that can be rapidly processed and directly viewed. This film is particularly useful for thoracic imaging. In addition, the radiographic film of this invention also has what is known as xe2x80x9cvisually adaptive contrastxe2x80x9d because it can provide higher contrast than normal in the higher density regions of an image. This invention also provides a film/screen imaging assembly for radiographic purposes, and a method of processing the film to obtain a high contrast black-and-white image.
Over one hundred years ago, W. C. Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913, Eastman Kodak Company introduced its first product specifically intended to be exposed by X-radiation (X-rays). Today, radiographic silver halide films account for the overwhelming majority of medical diagnostic images. Such films provide viewable black-and-white images upon imagewise exposure followed by processing with the suitable wet developing and fixing photochemicals.
In medical radiography an image of a patient""s anatomy is produced by exposing the patient to X-rays and recording the pattern of penetrating X-radiation using a radiographic film containing at least one radiation-sensitive silver halide emulsion layer coated on a transparent support. X-radiation can be directly recorded by the emulsion layer where only low levels of exposure are required. Because of the potential harm of exposure to the patient, an efficient approach to reducing patient exposure is to employ one or more phosphor-containing intensifying screens in combination with the radiographic film (usually both in the front and back of the film). An intensifying screen absorbs X-rays and emits longer wavelength electromagnetic radiation that the silver halide emulsions more readily absorb.
Another technique for reducing patient exposure is to coat two silver halide emulsion layers on opposite sides of the film support to form a xe2x80x9cdual coatedxe2x80x9d radiographic film so the film can provide suitable images with less exposure. Of course, a number of commercial products provide assemblies of both dual coated films in combination with two intensifying screens to allow the lowest possible patient exposure to X-rays. Typical arrangements of film and screens are described in considerable detail for example in U.S. Pat. No. 4,803,150 (Dickerson et al), U.S. Pat. No. 5,021,327 (Bunch et al) and U.S. Pat. No. 5,576,156 Dickerson).
One important component of the films described in these patents is a microcrystalline dye located in a silver halide emulsion layer or antihalation layer that reduces xe2x80x9ccrossoverxe2x80x9d (exposure of an emulsion from light emitted by an intensifying screen on the opposite of the film support) to less than 10%. Crossover results in reduced image sharpness. These microcrystalline dyes are readily decolorized during the wet processing cycle so they are not visible in the resulting image.
Radiographic films that can be rapidly wet processed (that is, processed in an automatic processor within 90 seconds and preferably less than 45 seconds) are also described in the noted U.S. Pat. No. 5,576,156. Typical processing cycles include contacting with a black-and-white developing composition, desilvering with a fixing composition, and rinsing and drying. Films processed in this fashion are then ready for image viewing. In recent years, there has been an emphasis in the industry for more rapidly processing such films to increase equipment productivity and to enable medical professionals to make faster and better medical decisions.
As could be expected, image quality and workflow productivity (that is processing time) are of paramount importance in choosing a radiographic imaging system [radiographic film and intensifying screen(s)]. One problem with known systems is that these requirements are not necessarily mutually inclusive. Some film/screen combinations provide excellent image quality but cannot be rapidly processed. Other combinations can be rapidly processed but image quality may be diminished. Both features are not readily provided at the same time.
In addition, the characteristic graphical plots [density vs. log E (exposure)] that demonstrate a film""s response to a patient""s attenuation of X-ray absorption indicate that known films do not generally provide desired sensitivity at the highest image densities where important pathology might be present. Traditionally, such characteristic sensitometric xe2x80x9ccurvesxe2x80x9d are S-shaped. That is the lower to midscale curve shape is similar to but inverted in comparison with the midscale to upper scale curve shape. Thus, these curves tend to be symmetrical about a density midpoint.
Another concern in the industry is the need to have radiographic films that as accurately as possible show all gradations of density differences against all backgrounds. It is well known that the typical response of the human eye to determining equal differences in density against a background of increasing density is not linear. In other words, typically it is more different for the human eye to see an object against a dark background than it is to see an object against a lighter background. Therefore, when an object is imaged (for example using X-rays, with or without intensifying screens) at the higher densities of the sensitometric curves, it is less readily apparent to the human eye when the radiographic film is being viewed. Obviously, this is not a desirable situation when medical images are being viewed and used for important diagnostic purposes.
In order to compensate for this nonlinearity of response by the human eye, it would be desirable to somehow increase radiographic film contrast only at the higher densities without changing contrast or other properties at lower densities. The result of such a modification would be a unique sensitometric curve shape where the contrast is higher than normal in the higher density regions. Such a curve shape is considered as providing xe2x80x9cvisually adaptive contrastxe2x80x9d (VAC).
While this type of sensitometry sounds like a simple solution to a well known problem, achieving it in complicated radiographic film/screen systems is not simple and is not readily apparent from what is already known in the art. Moreover, one cannot predict that even if VAC is obtained with a particular radiographic film, other necessary image properties and rapid processability may be adversely affected.
Thoracic radiographic imaging is intended to provide excellent images of radiotransparent as well as more difficult areas to penetrate as behind the heart, lungs, diaphragm, and mediastinum. However, because much of the pathology of interest is in the lungs, some radiologists have a preference for using a high contrast film and as such are willing to sacrifice seeing areas that are more difficult to see. Generally, the imaging requirements for such a film is high contrast in the toe or lower scale region of the characteristic sensitometric curve, and very high contrast in the mid- and upper scales. Conventional high contrast radiographic films tend to have similar curve shapes in both the lower and upper scale regions and therefore exhibit the traditional S-shape in the curve.
However, it may be that better films for thoracic imaging would have a higher contrast in the toe region while maintaining the conventional contrast in the mid- and upper scale regions. In addition, it would be desirable for such films to provide visually adaptive contrast (VAC) so that the fall off of sensitivity of the human eye can be compensated by higher peak gammas at higher densities. With these constraints in mind, the industry has been looking for a thoracic radiographic film that would provide good lung field information while also providing information in more radiopaque areas such as the mediastinum and retrocardiac regions. It is also desirable to have a radiographic film/screen combination that has the desired image quality, rapid processability, and high resolution.
The present invention provides a solution to the noted problems with a high contrast radiographic silver halide film comprising a support having first and second major surfaces and that is capable of transmitting X-radiation,
the film having disposed on the first major support surface, two or more hydrophilic colloid layers including first and second silver halide emulsion layers, and on the second major support surface, two or more hydrophilic colloid layers including third and fourth silver halide emulsion layers, the first and third silver halide emulsion layers being closer to the support than the second and fourth silver halide emulsion layers,
each of the first, second, third and fourth silver halide emulsion layers comprising silver halide tabular grains that (a) have the same or different composition in each silver halide emulsion layer, (b) account for at least 50% of the total grain projected area within each silver halide emulsion layer, (c) have an average thickness of less than 0.3 xcexcm, and (d) have an average aspect ratio of greater than 5,
all hydrophilic layers of the film being fully forehardened and wet processing solution permeable for image formation within 45 seconds,
the first and third silver halide emulsion layers comprising at least one particulate dye that is (a) capable of absorbing radiation to which the silver halide emulsions are sensitive, (b) present in an amount sufficient to reduce crossover to less than 15%, and (c) capable of being substantially decolorized during wet processing,
the first and third silver halide emulsion layers also comprising a rhodium dopant for the tabular silver halide grains, the rhodium dopant being present in each silver halide emulsion layer in an amount, independently, of from about 1xc3x9710xe2x88x925 to about 5xc3x9710xe2x88x925 mole per mole of silver in each emulsion layer,
the ratio of photographic speed of the first silver halide emulsion layer to the second silver halide emulsion layer and the ratio of the third silver halide emulsion layer to the fourth silver halide emulsion layer being independently greater than 0.3 logE, and
the film being capable of providing an image that has visually adaptive contrast and a peak gamma  greater than 3.1 while maintaining gammas  greater than 1.0 out into the toe of a sensitometric D vs. logE curve to a value of xe2x88x920.9 logE.
This invention also provides a radiographic imaging assembly comprising the radiographic film described above provided in combination with an intensifying screen on either side of the film.
Further, this invention provides a method of providing a high contrast black-and-white image comprising contacting the radiographic film described above, sequentially, with a black-and-white developing composition and a fixing composition, the method being carried out within 90 seconds to provide an image that has visually adaptive contrast and a peak gamma  greater than 3.1 while maintaining gammas  greater than 1.0 out into the toe of a sensitometric D vs. logE curve to a value of xe2x88x920.9 logE.
Thus, the present invention provides a high contrast radiographic film and film/intensifying screen assembly that gives the medical professional a greater ability to see an object against a dark (or high density) background. Therefore, when an object is imaged using the film of this invention at the higher densities, the object is more readily apparent to the human eye.
In order to compensate for the nonlinearity of response by the human eye, the radiographic film contrast has been increased only at the higher densities without changing contrast or other properties at lower densities. The result of such a modification is a unique sensitometric curve shape where the contrast is higher than normal in the higher density regions. Thus, the films of this invention are considered as providing xe2x80x9cvisually adaptive contrastxe2x80x9d (VAC).
Moreover, the film of this invention has specifically designed emulsion layers of specific photographic speeds to provide high contrast images, especially in the mid-scale to upper scale of the sensitometric curve while providing greater exposure latitude in the toe region. Thus, this film can be confidently used for thoracic imaging in the lung field while also providing information in more radiopaque areas such as the mediastinum and retrocardiac regions.
In addition, all other desirable sensitometric properties are maintained, crossover is desirably low, the images have high resolution, and the films can be rapidly processed in conventional processing equipment and compositions.